Incorrect initialization of mix_out in networks

[MassimoCimmino]

The initialization of _mix_out in Network._initialize_stored_coefficients() has incorrect shapes:

pygfunction/pygfunction/networks.py

Lines 852 to 853 in c796630

	self._mixing_m_flow = np.empty(self.nInlets)
	self._mix_out = np.empty((self.nBoreholes, 1))

The correct shape is (1, self.nBoreholes).

Also, using numpy.empty() can create problems since stored values are compared to check if calculation is necessary. numpy.empty() does not overwrite memory for the creation of the array and in some cases may present a realistic value for _mixing_m_flow at initialization while the _mix_out coefficient is incorrect. Their values should be overwritten with numpy.nan:

        self._mixing_m_flow = np.empty(self.nInlets)
        self._mixing_m_flow[:] = np.nan
        self._mix_out = np.empty((1, self.nBoreholes))
        self._mix_out[:] = np.nan